Summer Pay Experiences?... Good and Bad

Q: Why do gypsies have glass children?
A: Because they have crystal balls.
In fact I do have a crystal ball which I used to keep on my desk to consult ostentatiously when people asked me question like yours which were impossible to answer.
With one exception every employer I have worked for has paid final holiday pay up front. What does it say in your contract?
If the subject is not covered in the contract it might be worth talking to your HT and asking for your holiday pay before you leave as you are switching jobs/ countries you have some heavy upfront expenses. If the answer is 'no' make sure you have fulfilled all your obligations to the school and that everything you owe is paid up. This will remove any excuses for docking your final pay.
Before you leave it would be a good idea to find out the contact details of a local lawyer who deals with employment issues. You would then have somebody to contact if the Summer cash is not forthcoming.